1. Field of the Invention
The present invention relates generally to a mobile communication system employing Orthogonal Frequency Division Multiple Access (OFDMA), and in particular, to a system and method for dynamically allocating resources according to channel states.
2. Description of the Related Art
With the introduction of the cellular mobile communication system in the U.S. in the late 1970's, South Korea began to provide a voice communication service based on an Advanced Mobile Phone Service (AMPS) system, also referred to as a first generation (1G) analog mobile communication system. In the mid 1990's, South Korea commercialized a Code Division Multiple Access (CDMA) system, that is a second generation (2G) mobile communication system, to provide voice and low-speed data services.
Since the late 1990's, South Korea has partially deployed an IMT-2000 (International Mobile Telecommunication-2000) system, also known as a third generation (3G) mobile communication system, aimed at an advanced wireless multimedia service, global roaming, and a high-speed data service. The 3G mobile communication system was especially developed to transmit data at high rate to accommodate the rapid increase in the amount of data serviced therein.
Currently, the 3G mobile communication system is evolving into a fourth generation (4G) mobile communication system. In the 4G mobile communication system, referred to as a next generation communication system, active research is being conducted on technology for providing users with services guaranteeing various qualities of service (QoSs) at a data rate of about 100 Mbps. The current 3G mobile communication system generally supports a data rate of about 384 Kbps in an outdoor channel environment having a relatively poor channel environment, and supports a data rate of a maximum of 2 Mbps in an indoor channel environment having a relatively good channel environment.
A wireless local area network (LAN) system and a wireless metropolitan area network (MAN) system generally support a data rate of 20 to 50 Mbps. In the current 4G communication system, active research is being carried out on a new communication system that can provide for the mobility of a terminal and maintain a QoS for the wireless LAN system and the wireless MAN system supporting a relatively high data rate in order to support a high-speed service that the 4G communication system aims to provide.
When broadband spectrum resources are used to provide the high-speed data, for example a wireless multimedia service, intersymbol interference (ISI) occurs due to the multipath propagation. The intersymbol interference reduces the entire transmission efficiency of the system. Orthogonal Frequency Division Multiplexing (OFDM) has been proposed to resolve the intersymbol interference problem caused by the multipath propagation. OFDM is a technique for dividing the entire frequency band into a plurality of subcarriers before transmission. The use of OFDM increases one symbol duration, thereby minimizing the intersymbol interference.
OFDM, a modulation technique for transmitting data using multiple carriers, is a special case of the MCM (Multi-Carrier Modulation) technique in which an input serial symbol stream is converted into parallel symbol streams and then the parallel symbol streams are modulated into multiple orthogonal subcarriers before being transmitted. The first MCM systems appeared in the late 1950's for use in military high frequency (HF) radio communication systems, and the OFDM with overlapping orthogonal subcarriers was initially developed in the 1970's. In view of orthogonal modulation between multiple carriers, OFDM has limitations in actual implementation for systems. In 1971, Weinstein, et al. proved that OFDM modulation/demodulation can be efficiently processed using Discrete Fourier Transform (DFT), which was a driving force behind the development of OFDM. Also, with the introduction of a guard interval and a cyclic prefix as the guard interval further mitigates the adverse effects multipath propagation and delay spread have on systems. That's why OFDM has widely been exploited for digital transmission technologies such as digital audio broadcasting (DAB), digital TV broadcasting, wireless local area network (WLAN), and the wireless asynchronous transfer mode (WATM). Although the hardware complexity was an obstacle in the implementation of the OFDM, recent advances in digital signal processing technology including Fast Fourier Transform (FFT) and Inverse Fast Fourier Transform (IFFT) enable OFDM to be implemented.
OFDM, although being similar to the conventional Frequency Division Multiplexing (FDM), is distinguished therefrom in that the OFDM can secure optimal transmission efficiency during a high-speed data transmission by maintaining orthogonality between the subcarriers. In addition, OFDM is characterized in that it has a high frequency efficiency and is not significantly effected by multipath fading, thereby securing optimal transmission efficiency during high-speed data transmissions. Further, because OFDM uses overlapping frequency spectrums, it has high frequency efficiency, is not significantly effected by frequency selective fading and multipath fading, reduces intersymbol interference (ISI) using a guard interval, enables the design of an equalizer with a simple hardware structure, and is not significantly effected by impulse noises. Based on these advantages, OFDM is being actively applied to communication systems.
Orthogonal Frequency Division Multiple Access (OFDMA), OFDM-based Multiple Access, reconfigures some of the subcarriers from among all of the subcarriers as a subcarrier set, and allocates the subcarrier set to a particular access terminal (AT). OFDMA supports Dynamic Resource Allocation (DRA) capable of dynamically allocating a subcarrier set to a particular access terminal according to a fading characteristic of a wireless transmission line.
FIG. 1 is a diagram illustrating a configuration of a mobile communication system employing OFDMA (“OFMDA mobile communication system”). Referring to FIG. 1, the OFDMA mobile communication system, having a multi-cell configuration, i.e. having a cell 100 and a cell 150, includes an access point (AP) 110 for managing cell 100, an access point 160 for managing cell 150, an access router (AR) 120 for controlling the access points 110 and 160, access terminals (ATs) 111 and 113 for receiving a service provided from the access point 110, access terminals 161 and 163 for receiving a service provided from the access point 160, and an access terminal 131 that is in the process of being handed over to the access point 160 while receiving a service provided from the access point 110. It should be noted herein that the access router serves as a base station controller (BSC), and the access points serve as base stations (BSs). Signal transmission/reception between the access points 110 and 160 and the access terminals 111, 113, 131, 161 and 163 is achieved using OFMDA.
In order to increase channel efficiency between an access point and access terminals located in the same cell, resources must be shared. In the OFDMA mobile communication system, the subcarriers are the typical resources that can be shared by a plurality of access terminals, and the subcarriers are grouped into subcarrier sets. The entire transmission efficiency of the OFDMA mobile communication system is based on the allocation of the subcarriers to the access terminals located in the cell. That is, scheduling for the subcarrier allocation is always an important factor for improving the performance of the OFDMA mobile communication system. However, because the allocation of the subcarriers is determined according to the channel states, research is currently being conducted to devise a scheme for allocating subcarriers by accurately measuring a state of an allocated channel.
A description will now be made of a scheduling technique, or a technique for allocating the subcarriers.
Typically, the technique for allocating subcarriers is classified into a Static Channel Allocation (SCA) and a Dynamic Channel Allocation (DCA). SCA includes a Static Subcarrier Assignment (SSA), a Pseudo Static Assignment (PSA), and a Simple Rotating Subcarrier Space Assignment (Simple RSSA). DCA also typically includes a Fast Dynamic Channel Allocation (Fast DCA).
a. SSA
SSA, the simplest subcarrier allocation technique, allocates a fixed and predetermined number of subcarriers to each of the access terminals. That is, SSA allocates to a particular access terminal a fixed and predetermined number of subcarriers from among all of the subcarriers for the OFDMA mobile communication system regardless the channel states. Because SSA allocates the same number of subcarriers to all access terminals, it guarantees a fairness of the channel allocation but cannot guarantee channel the quality of the subcarriers allocated to the access terminals.
b. PSA
PSA mutually exchanges, between access terminals, the fixed and predetermined number of the subcarriers allocated to the access terminals, and reallocates the exchanged subcarriers. That is, PSA, although it allocates the same number of subcarriers to all access terminals, can prevent deterioration in the channel quality of the access terminals by exchanging the allocated subcarriers between the access terminals. PSA allocates subcarriers having a relatively higher channel quality to the access terminals, thereby increasing the entire transmission efficiency of the OFDMA mobile communication system.
c. Simple RSSA
Simple RSSA, a technique similar to PSA, allocates the same number, or the predetermined number of subcarriers, to all of the access terminals. However, Simple RSSA, unlike PSA, allocates subcarriers having higher channel quality to access terminals having higher priority, for example by taking into consideration a QoS level. Although Simple RSSA can guarantee fairness in terms of the number of allocated subcarriers, it cannot guarantee a fairness related to the channel allocation because it allocates channels to access terminals by considering the QoS level.
d. Fast DCA
Fast DCA minimizes intracell interference or intercell interference, and allocates subcarriers having the best channel quality to access terminals by taking into consideration the channel quality itself. That is, Fast DCA dynamically allocates subcarriers to access terminals according to the channel quality, thereby maximizing transmission efficiency of the OFDMA mobile communication system.
Also, active research is being conducted to devise a scheme for efficiently allocating sets of subcarriers, i.e. subchannels, to access terminals by taking into consideration the OFDMA characteristic so as to maximize user diversity. In the proposed scheme to efficiently allocate the subchannels to the access terminals, the use of channel quality information (CQI) being fed back to apply an Adaptive Modulation and Coding (AMC) to the access terminals is not restricted only to a physical layer but is extended to a medium access control (MAC) layer. In other words, the scheme for efficiently allocating the subchannels to the access terminals applies AMC based on CQI fed back from an access terminal, i.e. allocates a Modulation and Coding Scheme (MCS) level to a corresponding access terminal in the physical layer, and dynamically allocates subchannels using the CQI in the MAC layer. Therefore, in order to maximize the transmission efficiency of the OFDMA mobile communication system, a scheme for determining in which layer to apply the AMC and DCA must also be taken into consideration.
FIG. 2 is a diagram illustrating a timing relation in the case where AMC and DCA are applied according to a decision made by an access point in a general OFDM mobile communication system. Referring to FIG. 2, an access terminal 200 transmits CQI to its access point 220 for a predetermined CQI transmission period 204 (in step 202). One example of the CQI is a signal-to-noise ratio (SNR). The access point 220 applies AMC and DCA to the access terminal 200 based on the CQI transmitted from the access terminal 200. The access point 220 determines an MCS level to be applied to the access terminal 200 and allocates a subchannel to the access terminal 200 based on CQI transmitted from the access terminal 200 (in step 222). In this case, the access point 220 selects the best subchannel for the access terminal 200 to use from among the idle subchannels based on the CQI transmitted from the access terminal 200. Although not illustrated in FIG. 2, the access point 220 transmits information on the allocated MCS level and the subchannel to the access terminal 200. Then the access terminal 200 communicates with the access point 220 through the allocated subchannel according to the MCS level.
In the case where AMC and DCA are applied according to a decision made by the access point 220 as described above, and because the access point 220 allocates an MCS level and a subchannel to be used by the access terminal 200, a back-haul delay time required in a network can be minimized and an MCS level and a subchannel can be correctly allocated by taking into consideration the channel state of the access terminal 200.
However, as illustrated in FIG. 2, when the access terminal 200 performs a handover, the access point 220 must transmit to an access router 240 the information required to perform the handover of the access terminal 200 (in step 224). The access router 240 performs the handover process such that the access terminal 200 can be handed over from the access point 220 to another access point (not shown), based on the handover process information for the access terminal 200, transmitted from the access point 220 (in step 244), and transmits to the access point 220 the handover process information based on the handover process (in step 226). Then the access point 220 performs a handover-related procedure for the access terminal 200 using the handover process information transmitted from the access router 240 (in step 230).
In case of the handover, because the access point 220 performs the handover procedure for the access terminal 200 not by itself but in cooperation with the access router 240, a delay time occurs. The delay time includes a transmission time 242 required to transmit the handover process information from the access point 220 to the access router 240, and a transmission time 228 required to transmit the handover process information to the access point 220. A delay time corresponding to the time required for the handover process to occur, and the occurrence of the delay time, obstructs the fast handover process of the access terminal 200. When the access point 220 transmits a packet to the access router 240 to perform the handover, in some cases, a transmission overlap occurs between the packets during the handover process. Because packets are occasionally lost, in the case where DCA and AMC are applied according to a decision made by the access point 220 as illustrated in FIG. 2, the transmission packets must include their unique serial numbers before being transmitted. Undesirably, however, the transmission of the serial numbers causes a reduction in transmission efficiency.
A process of applying AMC and DCA according to a decision made by an access point in an FODM mobile communication system has been described so far with reference to FIG. 2. Next, with reference to FIG. 3, a description will be made of a process of applying AMC and DCA according to a decision made by an access router in an OFMD mobile communication system.
FIG. 3 is a diagram illustrating a timing relation in the case where AMC and DCA are applied according to a decision made by an access router in a general OFDM mobile communication system. Referring to FIG. 3, an access terminal 300 transmits CQI to its access point 320 during a predetermined CQI transmission period 304 (in step 302). One example of the CQI is an SNR. The access point 320 transmits to an access router 340 the CQI received from the access terminal 300 (in step 322). Then the access router 340 applies AMC and DCA to the access terminal 300 for an access router's processing time 344 and a scheduling time 346 based on the CQI from the access terminal 300 transmitted from the access point 320. That is, the access router 340 allocates an MCS level and a subchannel to be applied to the access terminal 300 based on the CQI received from the access terminal 300.
In the case where AMC and DCA are applied according to a decision made by the access router 340 as described in connection with FIG. 3, a back-haul delay time in a network occurs. The back-haul delay time includes a CQI transmission time 342 from the access point 320 to the access router 340 and a transmission time 306 required when information on the MCS level and the subchannel allocated by the access router 340 is transmitted to the access point 320. As stated above, the back-haul delay time in a network does not take into consideration the CQI from the access terminal 300 on a real-time basis, i.e. does not correctly consider a channel state of the access terminal 300, thereby reducing reliability on MCS level and subchannel allocation by the access router 340.